Fracturing is widely used in oil and gas production operations as a means of increasing the productivity of wells completed in oil field formations. This process ordinarily involves forcing liquids into the well under high pressures which open cracks in the formations surrounding the well. While maintaining pressure, solid particulate materials are introduced into the freshly formed fractures to prop them open so the fractures will not close or heal after the pressure has been removed.
Sand is frequently used as a propping agent in hydraulic fracturing operations applied to oil wells. Sand is relatively inexpensive and, when suspended in a suitable carrier liquid, can readily be introduced into the fractures. Ordinary sand can be used as a propping agent except in special situations. For example, in very deep producing formations, stronger propping agents are required because the greater overburden pressure tends to crush certain common materials including most sand available for this purpose. In other situations, when sand is used as a fracture proppant in formations being stimulated by steam and/or alkaline fluid injection, the hot alkaline fluids dissolved the sand, allowing the fractures to heal.
Another production problem frequently encountered in oil production activities involves the packing of particulate matter including sand or gravel into areas adjacent to an oil production formation to form what is commonly referred to in the industry as a "gravel pack", which is a permeable sieve around the production well. This screen permits passage of fluids therethrough while restraining the flow of unconsolidated sand into the formation. Sand flow into a producing well can cause serious and costly problems because the well will sometimes fill completely with sand and cease producing. In other instances, the sand produced from the formation flows to the surface of the earth and causes very high rates of wear in pumps and other mechanical goods through which the produced fluid passes. It is common practice to perform procedures in producing wells completed in formations containing unconsolidated sand to restrain the flow of sand from the formation into the well, and one of the commonly used procedures involves placing sand or gravel inside of metal screens attached to the production tubing, which functions as an effective filter for unconsolidated particulate matter from the formation.
When viscous oil containing formations are stimulated thermally, as by steam flooding, serious problems develop in connection with the use of ordinary uncoated sand in fracturing operations or in sand packed screens used to control unconsolidated sand flow from formations. Very often, caustic substances are used in combination with steam or other heated fluids to stimulate production of viscous petroleum. Although sand is ordinarily considered to be inert, it is eroded or slowly dissolved by the passage of hot, high pH fluids through the sand packed areas when fluids are recovered from oil formations being stimulated by steam flooding. The result is that the sand utilized in either as a proppant in fracturing operations or as the pack in a gravel or sand pack completion is slowly dissolved, until it no longer functions either as a proppant or as a filter medium in the sand packed screen. In order to eliminate this problem, it is necessary to utilize some material more resistant than sand to the high pH environment present when fluids are being recovered from formations during steam flooding or to coat sand with polymeric material which increases the resistance to high pH fluids such as are encountered in steam flooding operations. Although techniques have been disclosed in the literature for the purpose of coating sand, most procedures are intended to increase the physical strength of the sand in order to permit its use in fracturing operations in very deep wells. The coating which solves that problem is not effective for reducing the sensitivity of sand to high pH fluid flow conditions such as are described above. Accordingly, it can be appreciated that there is a substantial unfulfilled need for a method for forming polymer coated sand or other aggregate for use in propping fractures formed in formations being subjected to thermal stimulation, or for use in forming sand packs or gravel packs to reduce sand flow from formations being stimulated by thermal methods.